Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes, for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons, that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets and pole pieces have been tried in an attempt to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are often arranged in a sequence of alternating magnetizations, either parallel to, or anti-parallel to, the direction of the electron flow. The magnets and pole pieces are annular in shape and their axes are aligned with the path of the electron beam. The pole pieces, constructed of ferromagnetic material such as electrolytic iron, are placed between the magnets and provide a path through which magnetic flux may leak out of the structure as well as a path through which magnetic flux from the magnets may be directed into the working space along the axis of the traveling wave tube in order to influence the beam in the desired manner.
In addition to traveling wave tubes, other devices employ magnetic structures to manipulate charged particle beams. For example, the "wiggler" and the "undulator" use a shaped magnetic field to produce electromagnetic radiation by accelerating the charged particles in directions perpendicular to the path of the beam.
One of the critical problems confronting those who develop magnetic structures used to manipulate beams of charged particles has been how to more efficiently use the magnetic materials which make up the structure. The specific problems include: how to maximize the strength of the magnetic field along the path of the charged particle beam without increasing the weight of the structure, and how to increase the amount of useful flux along the path of the beam by decreasing the percentage of leakage flux. The present invention addresses these problems.